Measuring stress intensity factors during fatigue crack growth using thermoelasticity

Thermoelastic stress analysis has been developed in recent years as a direct method of investigating the crack tip stresses in a structure under cyclic loading. This is a consequence of the fact that stress intensity factors obtained from thermoelastic experiments are determined from the cyclic stress field ahead of a fatigue crack, rather than inferred from measurement of the crack length and load range. In the present paper the results of fatigue crack growth tests performed on welded ferritic steel plates are reported. From the results it can be observed that the technique is sensitive to the effects of crack closure and the presence of tensile and compressive residual stresses due to welding.     

Application of Digital Image Correlation to Address Complex Motions in Thermoelastic Stress Analysis

A motion compensation method for thermoelastic stress analysis (TSA) is described that uses digital image correlation (DIC) to capture the displacement field on the surface of the specimen. The displacement field is used to correct the infrared (IR) images to remove the effect of the motion of the specimen from the TSA. As the DIC displacements are obtained with a relatively high spatial resolution, sharp displacement gradients and discontinuities can be corrected. The feasibility of the motion compensation method for TSA is investigated firstly by validating the approach using data obtained from an aluminium alloy plate with a central circular hole loaded in tension and comparing the results with a finite element model. It is shown that the motion compensation approach significantly improves the accuracy of TSA, particularly when high magnification optics are used. Next, the feasibility of simultaneous capture of IR and white light images is investigated. It is shown that by using the correct combination of paints, a speckle pattern can be applied to the surface to provide contrast in the white light spectrum for the DIC but have a uniform emissivity in the IR spectrum so that there is no effect on the TSA. Thus, it is possible for the motion compensation to be conducted on data collected during fatigue tests. Finally, it is demonstrated that the motion compensation technique can be applied to discontinuous motion produced by face sheet debonding in a foam cored sandwich structure loaded in a double cantilever beam (DCB) configuration. It is shown that the motion compensation technique is capable of correcting the complex and non‐uniform motion for TSA in the DCB test, thereby enabling detailed thermoelastic data to be obtained from the vicinity of the crack tip.     

50th Anniversary Article: Seeing Stresses through the Thermoelastic Lens—A Retrospective and Prospective from an Australian Viewpoint

Thermoelastic stress analysis (TSA) has been around for the past 30 years, but to date, it is still a very much underrated and under‐utilised experimental technique. Although there are devoted groups of practitioners in some industries, this technology is not well known within the aerospace sector. In contrast, the Aerospace Division of the Defence Science and Technology Organisation (DSTO) in Australia has been in the forefront of this technology for some time, achieving many pioneering feats. This paper gives a brief introduction to the development of this technology from a historical perspective, then focuses on a number of innovations that have stemmed from DSTO, including the development and application of the world's first focal plane array based TSA system and, more recently, the development of small and robust microbolometer based systems. For the latter, it is shown that despite nominally poorer temperature sensitivities, they make ideal TSA devices and can in some cases outperform their much more expensive photon detector counterparts. Because of this, together with the enormous practical advantages of microbolometers, the future of TSA is shown to be brighter than ever. Specifically, it is argued that such TSA systems can play a major role in the pervasive and persistent surveillance of full scale fatigue testing of aircraft structures. By detecting both design and developing faults early, it can effectively relieve cost and schedule penalties that are often associated with unanticipated failures. To realise this capability, integration of this technology with autonomous systems will be important, and some preliminary but promising results from a technology demonstrator program are presented.     

The analysis of thermoelastic isopachic data from crack tip stress fields

A computer program—FACTUS (fracture analysis of crack tips using SPATE)—has been developed for the efficient analysis of thermoelastic data obtained from around a crack tip. The program is based on earlier work for the determination of stress intensity factors (SIFs), and also includes a novel solution procedure for the derivation of the non-singular stress term σ _{0 x} . The program has been used in the analysis of a series of large plate specimens with central or edge slots/cracks. The derived SIFs are compared with independent values. Issues, e.g. crack closure and the extent and effect of the plastic zone, are discussed.     

Beginnings and Early Development of Thermoelastic Stress Analysis

Abstract:  Starting with Gough's first recorded observation of the thermoelastic effect in 1803, through to the 1990s, the principal steps in the evolution of the thermoelastic stress analysis (TSA) technique are identified and reviewed. The coverage includes the contributions of Weber, who first described the effect in metals, Kelvin who provided the classical theoretical treatment, Turner and Coker who saw the potential of the effect as the basis of a stress analysis technique, Belgen who demonstrated the value of infrared technology in this context and Mountain and Weber whose foresight and skills resulted in the first commercially available equipment (SPATE) for TSA. Comments on the personal qualities and attributes of those involved are also included.     

基于有限元法和锁相热像法对含缺陷构件的应力分析与疲劳性能评估

基于有限元法研究含盲孔缺陷构件的应力集中系数Kt随盲孔深度h和盲孔直径的变化规律。利用锁相热像法的热弹性分析模式(E-Mode)研究盲孔附近的应力分布,预测不同深度盲孔的Kt,与有限元结果相比较发现吻合良好。通过Altair Li软件中的耗散模式(D-Mode)和Altair软件分别研究构件在疲劳过程中的固有耗散量和温度信号的变化规律,以评估疲劳损伤的演化过程。以固有耗散和温度信号的变化规律作为疲劳损伤的指标,快速预测带盲孔试件的疲劳极限,进而预测试件的疲劳缺口系数Kf。理论计算的结果证明了锁相热像法的有效性。     

Assessment of fatigue damage evolution in woven composite materials using infra-red techniques

Thermoelastic stress analysis (TSA) is used to study the growth of fatigue damage in single and two ply, 2 × 2 twill woven composite materials. Test specimens were subjected to a uniaxial tensile cyclic loading with maximum stresses of 10%, 15% and 20% of the ultimate failure stress. The development of fatigue damage locally within the weft yarns is monitored using high resolution TSA. The specimens were subsequently inspected using optical microscopy to evaluate the location and extent of cracks. Cracks were found in the weft fibres, running transverse to the loading direction. It is demonstrated that the lighter weight fabric is more resilient to damage progression. A signature pattern is identified in the TSA phase data that indicates the onset and presence of fatigue damage in the composite material.     

三维编织复合材料的微结构与力学性能研究进展

综述了国内外近年来对三维编织复合材料细观结构力学模型和力学性能的研究进展.细观结构力学模型研究主要是通过分析三维编织体的拓扑结构建立力学分析模型,主要有"米"字枝状模型、纤维倾斜模型和三细胞模型.对力学性能的研究包括刚度性能和强度性能研究,刚度性能的预测分析方法有解析法、基于均匀化理论的方法和有限元数值仿真;而强度性能的预测大多采用有限元方法来进行.最后,总结了目前研究中存在的问题并对以后的工作进行了展望.     

孔隙率对碳纤维/环氧树脂复合材料层合板湿热性能的影响

针对某飞机上应用的典型铺层[(±45)4/(0,90)/(±45)2]S和[(±45)/(0,90)2/(±45)]S,研究了孔隙对碳纤维增强环氧树脂基复合材料层合板的吸湿行为和层间剪切强度的影响。采用不同的热压罐固化压力制备了不同孔隙率的试样。采用显微图像分析技术对孔隙率和孔隙的微观结构特征进行了详细的分析。研究结果表明:孔隙主要分布于层间,且随着孔隙率的增大,孔隙的尺寸增大;2种层合板的吸湿率和最大吸湿量随着孔隙率的增加而增加;湿热老化和未经湿热老化的层间剪切强度都随着孔隙率的增加而下降。铺层[(±45)4/(0,90)/(±45)2]S未经湿热和湿热后的层间剪切强度随着孔隙率增加分别下降6%(孔隙率:0.6%~6.3%)和9%(孔隙率:0.4%~7.0%);铺层[(±45)/(0,90)2/(±45)]S未经湿热和湿热后的层间剪切强度随着孔隙率增加分别下降14%(孔隙率:0.4%~6.9%)和7%(孔隙率:0.2%~8.9%)。     

Through thickness stress distributions in pultruded GRP materials

This paper describes an experimental methodology for determining the through thickness properties of pultruded GRP materials and their application in finite element analysis (FEA) of adhesively bonded joints. The finite element analysis is validated using an infra-red thermography based experimental mechanics technique known as thermoelastic stress analysis. The obtained results show that the measured through thickness values fall within the assumed bounds of previous work and have highlighted that interactions between the fibres and resin in the through thickness direction are present but not in a particularly intuitive manner. Moreover, the work presented herein highlights that the value of shear modulus used in the numerical model is an important consideration.     

The Use of a Second Harmonic Correlation to Detect Damage in Composite Structures Using Thermoelastic Stress Measurements

Abstract: A novel development of the thermoelastic stress measurement technique using the second harmonic of the load test frequency is presented. Clear identification of significant sub-surface damage is demonstrated using the new technique, and test results are presented from a small scale wind turbine blade test which indicate that some quantitative assessment of damage criticality might be possible from a single full-field measurement. A hypothesis is presented as to why the new technique is successful, based on complex movements of the surface caused by local buckling (itself a consequence of the sub-surface damage). Strain gauge evidence is presented to support this hypothesis.     

复合材料层合板多尺度交互渐进损伤分析

纤维增强复合材料强度的准确表征是复合材料力学性能研究的核心问题之一。该文以碳纤维增强树脂基复合材料层合板为研究对象,基于宏观-细观多尺度分析方法,根据复合材料的物理失效模式分别给出了基体和纤维的细观失效准则,同时考虑基体失效对复合材料层合板纤维轴向力学性能的影响。提出了新的刚度退化方式,可准确表征复合材料层合板的损伤演化过程,开展了复合材料层合板四点弯模型的多尺度交互渐进损伤分析和试验验证。结果表明：基于多尺度方法的复合材料层合板宏-细观交互渐进损伤分析结果与试验结果吻合较好,新的刚度退化方式可以准确模拟层合板的失效过程。     

The influence of fiber-surface treatment on the mechanical properties of jute-polypropylene composites

This article concerns the effectiveness of MAH-PP copolymers (graft copolymer of PP and maleic anhydride) as coupling agents in jute-polypropylene composites. The fiber treatment time and the MAH-PP concentration influenced the mechanical properties of the composites. Flexural strength of the composites with MAH-PP treated fibers was higher than that of unmodified fibers, and increased with fiber loading. The cyclic-dynamic values at an increasing load indicated that the coupling agent reduces the progress of damage. Dynamic strength (dynamic failure stress at load increasing test) of the MAH-PP modified composites is therefore raised by about 40%. SEM investigations confirm that the increase in properties is caused by improved fiber-matrix adhesion. There was less inclination for fibers to pull out of the matrix.     

Poisson's ratio as a sensitive indicator of (fatigue) damage in fibre-reinforced plastics

Even if the extent of damage in fibre‐reinforced plastics is limited, it already affects the elastic properties. Therefore, the damage initiation and propagation in composite structures is monitored very carefully. Beside the use of nondestructive testing methods (ultrasonic inspection, optical fibre sensing), the follow‐up of the degradation of engineering properties such as the stiffness is a common approach. In this paper, it is investigated if the Poisson's ratio can be used as a sensitive indicator of (fatigue) damage in fibre‐reinforced plastics. Static, cyclic and fatigue tests have been performed on [0°/90°]_2s glass/epoxy laminates, and axial and transverse strain were measured continuously. The evolution of the Poisson's ratio ν_xy versus time and axial strain ɛ_xx is studied. It is concluded that the degradation of the Poisson's ratio can be a valuable indicator of damage, in combination with the stiffness degradation.